The details of Clarisse's death are suspicious, and because Montag doesn't witness it himself, readers are forced to rely on the narration of Mildred.

Clarisse and her family represent a threat to the government because of their nonconformist views. They don't spend hours in front of the parlor walls each evening and instead talk to each other—though initially, Montag can't imagine what they find to discuss. Clarisse tells him about her uncle, who once drove only forty miles an hour so that he could better observe the world around him; he was jailed for two days for this choice. Clarisse's curiosity and energy is juxtaposed to Mildred's attempted suicide, and Montag begins to truly appreciate the way that Clarisse makes him think when he compares her energy to his wife's grim existence.

Montag continues meeting with Clarisse and enjoys their conversations. Yet one day, he finds that she has disappeared. Montag immediately feels that something is "the matter." He asks his wife if she has seen the "high-school girl" next door, and Mildred replies that she is "gone":

McClellan. McClellan, Run over by a car. Four days ago. I'm not sure. But I think she's dead. The family moved out anyway. I don't know. But I think she's dead.

It's highly unlikely that Mildred had anything to do with Clarisse's death; she simply doesn't think about anything with enough passion to commit murder. Mildred is barely existing in her own life and doesn't even think Clarisse's death is worth mentioning.

It's possible that Clarisse has indeed been killed by someone, however, because her ideology conflicts with the government's. It's also possible that she has been intentionally killed by kids who are out joyriding in their misguided efforts to connect to any emotion at all. Clarisse herself points out the dangers she sees in kids her age:

I'm afraid of children my own age. They kill each other. Did it always used to be that way? My uncle says no. Six of my friends have been shot in the last year alone. Ten of them died in car wrecks.

It is likely that either the government or intentionally reckless teenagers killed Clarisse—not Mildred.

What does Mildred believe happened to Clarisse?

When Montag hasn't encountered Clarisse for a few days, he asks Mildred if she's happened to see her. Mildred says no. However, she also says that Clarisse's family has moved away and that she believes Clarisse is dead. Mildred states that she thinks Clarisse was run over by a car.

When Montag presses Mildred as to whether she knows for a fact that Clarisse is dead, Mildred says she isn't sure. She is certain the family moved but is only "pretty sure" that Clarisse is dead.

While we don't know that Clarisse has actually been run over by a car, it makes sense, given that Clarisse is given to taking walks, a very odd habit in this particular culture. Beatty tells Montag that Clarisse is better off dead because she was such an oddity, but Montag is upset by her death.